Imagine that everytime the mob swings at you, there is a table of possible results and a number from 1 to 100 is rolled. The number rolled is matched to the table entry, and that entry is the result of the attack. For example, a simple combat table might look like:
26-40 Crush (150% damage)
21-25 Critical (200% damage)
This is basically what happens in WoW when a mob attacks you. As you increase your defensive stats, the chances of each event change. If you have 490 Defense, criticals disappear from the table. As the block/parry/dodge/miss entries grow, they "push off" the upper entries.
So after getting a lot of tanking gear, the table might look like this:
91-100 Crush (150% damage)
Notice how hits have been pushed off the table entirely? And now you only have a 10% chance to get crushed?
If you got 10% more block/parry/dodge/miss, crushes would be pushed off the table entirely. You'd still take damage, because you take damage everytime you block. But you wouldn't take non-blocked hits, crushes, or criticals.
Because crushes are the last things to be pushed off the table, achieving this state is called being "uncrushable".